constructionroof
I'm making a carport that is about 10 feet wide with 2×6's for the "roof frame" and 4×6 posts. I was going to use a medal roof using tin I think. Do I first need to put down wood sheathing? Or can I attach the medal directly to the 2×6's?
I think the UNDERSIDE would look better seeing the medal but I also don't want to have it blow away.
You could easily use something like this.
It's often called a metpost or metal fence spike, depending on your region.
You can do a hot roof and not require any ventilation, though I would not recommend it anywhere snow accumulates on shingle roofs. Even if i don't like it, it is allowable.
Whether you need additional ventilation depends on how much you are getting now and what is allowed by the governing building code for your area. According to the International Building Code for One and Two Family Dwellings, R806 Roof Ventilation, you need the equivalent of 1/300 of the area ventilated in free vent area. If the soffit vents do not provide this, you need to install additional ventilation or opt for the hot roof concept.
Also, the attic area needs to accessible by a min 22x30 inch opening. You must also install a class 1 or II vapor retarder on the warm side of the insulation or the ventilation requirements double.
The building code does not address moisture control within the garage space, it's only concerned with moisture in concealed spaces or general ventilation of habitable spaces. You may wish to address moisture in the garage itself with exhaust fans or operable openings. This is unrelated to attic ventillation requirements.
Best Answer
I would not add plywood under the metal. The only thing that will come with this is the possibility of rot and the loosening of your metal sheet connections. However plywood does offer structural support to your roof so you may have to add a few crosses and make sure you have a good screw schedule on the tin sheets.